Printed circuit boards having a high density of electrical interconnections and circuitry have been developed from multilayer printed circuit board laminates to meet the needs for miniaturized electronic components. The multilayer printed circuit board laminates are made from suitable reinforced resin materials and thin metal foils, particularly copper foils, which are processed together at high temperature and pressure, such laminates being used in the manufacture of printed circuit boards.
A printed circuit board is made from a laminate by etching away portions of the conductive foils from the laminate surface to leave distinct patterns of conductive lines and formed elements on the surface of the etched laminate. Additional laminates and/or laminate materials are then packaged together with the etched product to form a multilayer circuit board package. Additional processing, such as hole drilling and component attaching, completes the printed circuit board package.
While printed circuit boards are well known, printed circuit board technology continues to advance to provide higher density boards with smaller printed circuit lines. Miniaturization has advanced to the point, accordingly, that surface contamination in storage and handling has now become a significant problem in addition to the problem of protecting the surfaces from the effects of high-temperature and high-pressure lamination processes.
During the high-pressure and high-temperature lamination of the thin copper foils, thin plastic lamination mold release coating films or layers have been successfully temporarily interposed on the foil to minimize deleterious effects upon the foil surfaces during such lamination, as described, for example, in Martin J. Wilheim, U.S. Pat. No. 4,753,847 of common assignee of the present invention. Among such removable interposed layers have been sheets of polyvinyl fluoride and the like, such as those of the "Tedlar" type of the Dupont Company.
As for protection during handling and otherwise before lamination, a clean environment can help or at least minimize, contamination problems.
Another way to avoid or minimize such problems is to use protective films as suggested in U.S. Pat. No. 5,120,590, involving edge adhesive attachment of such films over the foil.
The present invention, however, provides a vastly improved way to protect the surfaces of sheets from contamination during each of handling, shipping, storage and lamination, remarkably to minimize contamination problems in the manufacture of circuit boards and the like; and, at the same time, to resist damaging effects of the before-described high-temperature (about 350.degree. F.) and high pressure (about 500 to 1000 pounds/in.sup.2) lamination to other substrates.
This invention takes advantage, moreover, of the high temperature stability of bonding materials achieved by processing appropriate reactive pressure-intact adhesives to such plastic films, preferably by electron-beam curing. Thus, the present invention provides a foil laminate having a foil-protecting, electron beam-cured film or layer serving as a covering or coating which can be readily removed from the foil surface following the appropriate processing, and with no visible residue left on the foil. Furthermore, the adhesive coating used in this invention on these protective films can be applied with a high degree of uniformity across the entire surface of the film and then uniformly applied upon the foil, not just at edges or select regions, and without concern of film placement over the foil surface, nor of the effects of elevated temperature and pressure on the chemical structure of said adhesive, or any physical or chemical effects upon the foil surface.
By combining on one surface (outer) of the thin protective film covering or layer, the type of pressure-and temperature-resistive mold release-coating treatment described in before-mentioned U.S. Pat. No. 4,753,847, for example, and, on the other surface inner or (bottom), a suitable releasably attached cured pressure-sensitive adhesive coating against the foil surface, an attached protective layer or coating for that foil is provided during each of handling, shipping and storage, and a mold-release layer for lamination protection, as well.